Integrated circuit implementations of imaging arrays have undergone significant development over the past twenty years. One widely used technology in the pixels of imaging arrays is the charged coupled device (CCD). The CCD device has been made popular by its oftentimes superior performance characteristics. However, one drawback of the CCD image sensor is that signal processing elements cannot be manufactured using the same process that is used to manufacture CCD devices. Therefore, signal processing circuitry must be located in a separate integrated circuit.
Another class of image sensors are the active pixel sensors. As noted in U.S. Pat. No. 5,625,210 to Lee et al. ("the '210 patent), an active pixel sensor refers to electronic image sensors within active devices, such as transistors, that are associated with each pixel. The active pixel sensor has the advantage of being able to incorporate both signal processing and sensing circuitry within the same integrated circuit. Conventional active pixel sensors typically employ polysilicon photocapacitors or photodiodes as the active image sensing elements.
A device known as a pinned photodiode has gained favor for its ability to have good color response for blue light, as well as advantages in dark current density and image lag. While the '210 patent provides a method for using a pinned photodiode and an active pixel sensor, the design taught in the '210 patent suffers from the drawback of manufacturing complexity. In particular, as seen in the diagrams of the '210 patent, the manufacturer of such an apparatus requires multiple masking and photolithography steps.
Thus, what is needed, is a method of manufacturing a device having the advantages of an active pixel pinned photodiode as in the '210 patent with a minimum number of photolithography and masking steps. Alternatively stated, what is needed is a new pinned photodiode active pixel structure that can be more easily manufactured.